Method and apparatus to measure statistical variation of electrical signal phase in integrated circuits using time-correlated photon counting

Abstract

Time-correlated photon counting is used to measure integrated circuit (IC) performance related to signal jitter (such as clock jitter) in a manner that is non-invasive to the circuit or node of interest. The signal jitter is measured by counting photon emissions at various nodes of interest across a controlled collapse chip connect (C4) mounted die, without interfering with the normal operation of the circuit of interest. This increases the precision and accuracy of the measurement of signal jitter significantly, since small amounts of phase noise on a particular clock signal edge can be detected. The emitted photons can be detected and subsequently correlated to a precise time base to obtain a statistical spread of switching events in time. The range of the photon distribution can be used to reliably determine safe and reasonable timing guard bands for clock and data paths in an IC.

What is claimed is:  
     
       1. A method, comprising: 
       providing a signal to a device under test (DUT) to cause a circuit of the DUT to switch;  
       detecting a photon emitted from the DUT as a result of a switching event;  
       placing timing information corresponding to the detected photon in one time bin, among a plurality of time bins that together define a time window, to obtain a time-correlated photon emission count for that bin;  
       for that switching event, generating a single histogram based on photon emission counts in the bins in the time window; and  
       measuring a non-periodic statistical variation of the signal at the switching event based on the single histogram.  
     
     
       2. The method of  claim 1  wherein the time-correlated photon emission counts in each bin are obtained as a result of multiple switching events. 
     
     
       3. The method of  claim 1  wherein measuring the non-periodic statistical variation of the signal at the switching event includes obtaining a representation of a signal output of the DUT by performing a de-convolution technique using the single histogram and a transfer function of a detector that detected the emitted photon. 
     
     
       4. The method of  claim 3 , further comprising performing the de-convolution technique by taking into account a transit-time-spread of the detector and an inherent jitter. 
     
     
       5. The method of  claim 1  wherein measuring the non-periodic statistical variation of the signal comprises measuring a jitter or a skew of the signal. 
     
     
       6. The method of  claim 1  wherein the signal comprises a periodic clock signal. 
     
     
       7. The method of  claim 1  wherein measuring the non-periodic statistical variation of the signal at the switching event comprises: 
       determining an intrinsic variation of the signal by providing the signal to the DUT as a low activity pattern;  
       determining a dynamic variation of the signal by providing the signal to the DUT as a high activity pattern; and  
       determining an incremental variation by obtaining a difference in widths between a histogram associated with the low activity pattern and a histogram associated with the high activity pattern.  
     
     
       8. The method of  claim 7  wherein the low activity pattern comprises a jitter-free clock signal, and wherein the high activity test pattern comprises the clock signal and a jittery return signal from a clock distribution tree on the DUT, as output from an XOR gate coupled to the DUT. 
     
     
       9. The method of  claim 1 , further comprising performing a calibration procedure by summing a clock signal with a known noise signal as one input to an XOR gate coupled to the DUT, another input of the XOR gate being grounded. 
     
     
       10. The method of  claim 1 , further comprising: 
       emitting an analog signal to indicate time of arrival of the emitted photon, the analog signal indicative of a start signal;  
       generating a trigger signal indicative of a stop signal, the start and stop signals together defining a pulse having an amplitude corresponding the time of arrival of the emitted photon; and  
       digitizing the pulse to obtain a value of the amplitude and placing the value in a bin.  
     
     
       11. The method of  claim 1  wherein the time window comprises a tester loop, the method further comprising providing an acquisition time for a plurality of tester loops, each loop delimited by a reset signal. 
     
     
       12. An apparatus, comprising: 
       a photon detector positioned to receive a photon emitted from a device under test (DUT) in response to a signal that causes a circuit of the DUT to switch during a switching event; and  
       a data processing unit coupled to the photon detector to place timing information corresponding to the detected photon in one time bin, among a plurality of time bins that together define a time window, and to obtain a time-correlated photon emission count for that bin, wherein for that switching event, the data processing unit is capable to generate a single histogram based on photon emission counts in the bins in the time window and capable to measure a non-periodic statistical variation of the signal at the switching event based on the single histogram.  
     
     
       13. The apparatus of  claim 12  wherein the data processing unit includes a constant fraction discriminator circuit to process an analog signal generated by the photon detector to indicate arrival of the emitted photon. 
     
     
       14. The apparatus of  claim 12 , further comprising a time-to-amplitude converter (TAC) coupled to the data processing unit, the TAC capable to generate a pulse having an amplitude indicative of a time of arrival of the emitted photon, the data processing unit including an analog-to-digital converter to convert the amplitude of the pulse into a value corresponding to the time of arrival of the emitted photon, the data processing unit capable to place the value into the bin. 
     
     
       15. The apparatus of  claim 12 , further comprising a tester coupled to the DUT and to the data processing unit to provide the signal to the DUT, the tester further being capable to provide a reset signal to the data processing unit to reset the time window. 
     
     
       16. The apparatus of  claim 12 , further comprising: 
       a summing circuit to receive a clock signal at one input and a known noise signal at another input; and  
       an XOR gate having an input coupled to receive an output of the summing circuit, and having an output coupled to the DUT to provide the DUT with a signal having known jitter.  
     
     
       17. The apparatus of  claim 16  wherein another input of the XOR gate is grounded. 
     
     
       18. The apparatus of  claim 12 , further comprising an XOR gate having a first input coupled to receive a clock signal and having a second input coupled to receive a return signal from a clock distribution tree on the DUT, the XOR gate having an output coupled to the circuit of the DUT that switches during the switching event, wherein the data processing unit is capable to determine intrinsic variation in response to having the clock signal provided to the DUT by the XOR gate as a low activity pattern and capable to determine a dynamic variation in response to having the clock signal and the return signal provided to the DUT by the XOR gate as a high activity pattern. 
     
     
       19. The apparatus of  claim 18  wherein the data processing unit is capable to determine an incremental variation by obtaining a difference in widths between a histogram associated with the low activity pattern and a histogram associated with the high activity pattern. 
     
     
       20. The apparatus of  claim 12  wherein the photon detector comprises a photon-counting photomultiplier tube. 
     
     
       21. A system, comprising: 
       a tester to provide a signal to a device under test (DUT) to cause a circuit of the DUT to switch during a switching event;  
       a photon detector positioned to receive a photon emitted from the DUT during the switching event in response to the signal; and  
       a data processing unit coupled to the photon detector to place timing information corresponding to the detected photon in one time bin, among a plurality of time bins that together define a time window, and to obtain a time-correlated photon emission count for that bin, wherein for that switching event, the data processing unit is capable to generate a single histogram based on photon emission counts in the bins in the time window and capable to measure a non-periodic statistical variation of the signal at the switching event based on the single histogram.  
     
     
       22. The system of  claim 21 , further comprising a time-to-amplitude converter (TAC) coupled to the data processing unit, the TAC capable to generate a pulse having an amplitude indicative of a time of arrival of the emitted photon, the data processing unit including an analog-to-digital converter to convert the amplitude of the pulse into a value corresponding to the time of arrival of the emitted photon, the data processing unit capable to place the value into the bin. 
     
     
       23. The system of  claim 21 , further comprising: 
       a summing circuit to receive a clock signal at one input and a known noise signal at another input; and  
       an XOR gate having an input coupled to receive an output of the summing circuit, and having an output coupled to the DUT to provide the DUT with a signal having known jitter.  
     
     
       24. The system of  claim 21 , further comprising an XOR gate having a first input coupled to receive a clock signal and having a second input coupled to receive a return signal from a clock distribution tree on the DUT, the XOR gate having an output coupled to the circuit of the DUT that switches during the switching event, wherein the data processing unit is capable to determine an intrinsic variation in response to having the clock signal provided to the DUT by the XOR gate as a low activity pattern and capable to determine a dynamic variation in response to having the clock signal and the return signal provided to the DUT by the XOR gate as a high activity pattern. 
     
     
       25. The system of  claim 21  wherein the photon detector comprises a photon-counting photomultiplier tube.